1. Field
The inventive concept generally relates to image sensors and to methods of fabricating the same. The inventive concept also relates to backside illuminated image sensors, to CMOS image sensors, and to methods of fabricating the same.
2. Description of the Related Art
Image sensors that employ semiconductor devices for converting an optical image into electrical signals include charge coupled devices (CCD) and complementary metal oxide semiconductor (CMOS) image sensors. Pixels of a CCD device are provided by MOS capacitors which are biased to convert photons into electric charges carriers. The electric charge carriers are read out through the capacitors. On the other hand, pixels of a CMOS image sensor include photoelectric conversion devices which absorb incident light and accumulate electric charges corresponding to the amount of the incident light, and a number of MOS transistors associated with each photoelectric conversion device and which process the output of the photoelectric conversion devices.
Thus, the photoconversion devices and signal processing circuits in CMOS image sensors can be integrated in a single chip. Accordingly, compared to CCD image sensors, CMOS image sensors are smaller. In addition, CMOS image sensors are less costly to manufacture and consume less power. Therefore, CMOS image sensors are favored in products with limited battery capacity. For these reasons, the use of CMOS image sensors is expanding.
Circuits of a CMOS image sensor comprise a plurality of layers of metal wiring which output optical signals respectively stored in the photoelectric conversion devices and an interlayer insulating layer. Accordingly, the reflection of incident light by the metal wiring layers or the adsorption of the incident light by the interlayer insulating layer would reduce the sensitivity of the sensor. In addition, reflected light absorbed by adjacent pixels would cause crosstalk.
In light of this, a backside illuminated (BI) image sensor has been proposed. A BI image sensor includes a substrate having a front side to which the interlayer insulating layer containing the layers of metal wiring is attached. The backside of the substrate is polished, and is oriented such that the incident light impinges the backside of the substrate. Thus, in such a BI image sensor, incident light is not reflected by the metal wiring layers or absorbed by the interlayer insulating layer.